Airbag deployment monitor and sensing electronics

ABSTRACT

An airbag deployment sensor has a cartridge containing a quantity of tape one end of which is attached to the inside surface of an airbag cushion. Deployment of the cushion pulls tape from the cartridge at a rate that is monitored by transmitting light through the tape, or by detecting the presence of metalized, or magnetic shielding portions, of the tape.

FIELD OF THE INVENTION

The present invention relates to monitoring of airbag deployment with atape, and methods and circuits for processing a stream of data receivedfrom a sensor that monitors the rate at which the tape is beingwithdrawn from a cartridge.

BACKGROUND OF THE INVENTION

Experience has shown that airbags work best in combination with seatbelts and other safety systems. Although airbags contribute to theoverall safety of occupants of an automobile, they can present a dangerto a vehicle occupant who is positioned too close to an airbag when itdeploys. This condition, where the vehicle occupant is positioned sothat airbag deployment might be dangerous, is referred to as the vehicleoccupant being “out of position.” Various systems have been developed todetect an “out of position” vehicle occupant. Sensor systems designed todetect the vehicle occupant's position often require constant monitoringso that in the event of a crash the vehicle occupant's position isknown. Sensor systems designed to detect the position of the vehicleoccupant have been proposed based on ultrasound, optical, or capacitancesensors.

Constant monitoring of sensors, which may have high data rates, requiresthe design of algorithms which can reduce sensor data to a singlecondition or a limited number of data conditions which are used in anairbag deployment decision to prevent airbag deployment or for a duelstage airbag to select the level of deployment. Maintaining dataintegrity between the non-crash positional data, and positional dataneeded during airbag deployment is complicated by the noisy environmentproduced by a crash. Dealing with data integrity issues requiresincreased processor capabilities and algorithm development, which alsorequires additional testing.

Prior art approaches attempt to determine, based on various sensors, thedistance between the airbag and the passenger before the airbag isdeployed. In many instances, the vehicle occupant will not be too closeto the airbag at the time the decision to deploy the airbag is made,but, because of the rate at which the vehicle occupant is approachingthe airbag, the vehicle occupant will be too close when the airbag isactually deploying. To handle these situations, more sophisticatedsensors and algorithms are needed in order to attempt to predict thevehicle occupant's position when the airbag is actually deployed ornearly completely deployed. In other words, the ideal airbag deploymentsystem functions such that the airbag deploys fully or nearly fullybefore the vehicle occupant engages the airbag. Existing systems inhibitairbag deployment when, based on various sensors and algorithms, it isdetermined that, because of the position of the vehicle occupant, thebag is more likely to harm than to benefit the vehicle occupant.

Successfully creating a sensor and algorithm system is complicatedbecause there is usually very little delay between the decision todeploy and actual deployment. This is so because the maximum benefitfrom an airbag is achieved by early deployment, and at the same time,more time before deployment maximizes the information available todetermine whether deployment is necessary. The desire to maximizeeffective deployment of the airbag while minimizing unnecessarydeployment creates a tension between waiting for more information anddeploying immediately. Therefore, once sufficient information isavailable, deployment typically follows nearly immediately.

A system which employs vehicle occupant position sensors and algorithmsmust be able to supply at all times an indication of whether airbagdeployment should be inhibited so that the inhibit decision can beapplied whenever the airbag deployment decision occurs. This means thesensors and algorithms used to develop the vehicle occupant positioninhibit signal, cannot be optimized to deal with a specific time framein which the actual deployment decision is made. The end result is thatsuch algorithms may be less accurate than desired because they mustpredict events relatively far in the future—perhaps tens ofmilliseconds.

One known type of sensor shown in EP 0990567A1, employs a plurality oftapes which extend between the front of the airbag and a tape dispensingcartridge mounted on the airbag housing. Tape extraction sensors withinthe cartridge monitor the rate at which tape is withdrawn from thecartridge and thus can detect airbag impact with a vehicle occupant by adecrease in airbag velocity. This type of sensor which can monitor theway an airbag is actually deploying solves the problem of predictingwhether a vehicle occupant will be out of position at time of airbagdeployment. In this arrangement the airbag is deployed, and if itencounters a vehicle occupant before it has reached a certain stage ofdeployment the airbag is vented which effectively removes the airbag.Several tapes and tape dispensing cartridges are used to monitordifferent portions of the bag so that if any portion of the bag contactsa vehicle occupant, the fact of contact can be detected and the bagvented to prevent injury to the out-of-position occupant. To bepractical, this type of sensor—which monitors actual deployment—needssimple but robust techniques for monitoring the rate at which tape iswithdrawn from the cartridge.

SUMMARY OF THE INVENTION

The airbag deployment sensor of this invention has a cartridge in whicha quantity of tape is stored. One end of the tape is attached to theinside surface of an airbag cushion so that when the cushion is deployedit pulls tape from the cartridge. The rate at which the tape is pulledfrom the cartridge is monitored by transmitting light through the tape,or by detecting the presence of the metalized or ferrous portions of thetape.

In a first embodiment a tape ½ mm by 5 mm constructed of blackpolyethylene has 2 mm diameter holes spaced 5 mm on center extendingalong the length of the tape. An infrared light emitting diode ispositioned on one side of the tape and a phototransistor is positionedopposite the light emitting diode. The phototransistor is connected to acomparator circuit with hysteresis that provides a clean digital outputproportional to the rate at which the holes formed in the tape arepulled past the phototransistor. Alternatively, an infrared transparenttape on which an infrared opaque pattern has been printed may be used.

In a second embodiment, a tape ½ mm by 5 mm has 5 mm regions that arespaced 5 mm apart, which have been metalized. For example, a metal filmmay be deposited on Mylar® tape and selectively etched to form metalizedregions or metalized paint may be used on film or cloth. The metalizedregions may be detected by one of three methods. The first methodemploys two closely spaced contacts that are connected by the metalizedregions as they pass over the contacts. This type of detector may alsobe connected to a comparator circuit with hysteresis to provide adigital outlet. The second method for detecting the passage of themetalized regions employs a capacitive plate as a sensor. The capacitiveplate is part of an oscillator circuit where the frequency of theoscillator circuit is controlled by the capacitance of the capacitiveplate. As the metalized regions move opposite the capacitor plate, avariable capacitor is formed so that the amount of capacitance in thecircuit changes. With this varying capacitance, the frequency of theoscillator increases and decreases as the metalized regions pass thecapacitor plate. A third method of detecting the rate at which a tapewith metalized regions is pulled from the cartridge employs an amplitudemodulated signal. An oscillator of a few hundred kHz to about 1 MHz isconnected into a first electrode. A second electrode spaced from thefirst electrode is connected to an amplification circuit. The metalizedregion forms a capacitive link between the first electrode and thesecond electrode that efficiently transmits the oscillator signal to theamplifier. Therefore as the metalized regions pass the first and secondelectrodes, the signal received by the amplifier circuit varies inamplitude. The output of the amplifier is rectified, producing a pulsedDC output.

If the metalized region is formed from a ferromagnetic alloy, movementof the ferromagnetic region can be used with a permanent magnet toaffect a magnetic field sensor such as a Hall effect sensor, a GMRsensor, or even a simple conductor loop or coil. The permanent magnet ispositioned opposite the magnetic field sensor, and the ferromagneticmetalized region acts as a magnetic shield selectively blocking magneticfield lines from the permanent magnet to the magnetic field sensor.

It is a feature of the present invention to provide a tape that is drawnfrom a cartridge to detect airbag cushion employment rate that isconstructed to reliably affect a sensor.

It is a further feature of the present invention to provide methods fordetecting the velocity of a tape being pulled from a cartridge by anairbag cushion.

It is another feature of the present invention to provide a tape sensorcombination that employs detecting a change in capacitance.

It is a still further feature of the present invention to provide atape, sensor combination that employs detecting a change in magneticfield strength.

It is another feature of the present invention to remind a tape sensorarrangement that can accommodate variations in sensor performance due todevice to device variation and aging effects.

Further features and advantages of the invention will be apparent fromthe following detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a tape cartridge employing theinfrared sensor arrangement of this invention.

FIG. 2 is an electrical schematic drawing of a circuit used in the tapecartridge of FIG. 1 to provide a clean digital output from the infraredsensor.

FIG. 3 is a top plan view of a tape for use in the tape cartridge ofFIG. 1.

FIG. 4 is a top plan view of an alternative tape for use within the tapecartridge of this invention.

FIG. 5 is a schematic plan view of an alternative tape cartridge of thisinvention employing a capacitive sensor.

FIG. 6 is an electrical schematic drawing of a circuit used in the tapecartridge of FIG. 5 to provide a frequency modulated output signalproportional to the speed of the tape leaving the tape cartridge.

FIG. 7 is another electrical schematic drawing of a circuit used in thetape cartridge of FIG. 5 to provide an amplitude modulated output signalproportional to the speed of the tape leaving the tape cartridge.

FIG. 8 is a schematic plan view of a tape cartridge of this inventionemploying a magnetic field sensor.

FIG. 9 is an electrical schematic drawing of a circuit used in the tapecartridge of FIG. 8.

FIG. 10 is a schematic plan view of a yet further tape cartridge of FIG.1 employing a contact sensor.

FIG. 11 is a schematic cross sectional view of an airbag moduleincorporating the tape cartridge of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring more particularly to FIGS. 1-11, wherein like numbers refer tosimilar parts, an airbag housing 12 with an attached folded airbagcushion 13 is shown in FIG. 11. A gas generator 14 is mounted to theairbag housing which incorporates a valve 15 which can be used to stopthe inflation of the airbag cushion 13 by venting gas from the gasgenerator 14. As shown in FIG. 1, a cartridge 20 containing a length oftape 22 is mounted to the housing 12. As shown in FIG. 11, one end ofthe tape 22 is attached to the inside surface 18 of the airbag cushion13 so that when the gas generator 14 is activated and the airbag cushion13 is deployed, tape 22 is withdrawn from the cartridge 20.

As shown in FIG. 3, the tape 22 is a black polyethylene strip about 5 mmin height and about ¼ mm thick. The tape 22 may alternatively be formedof opaque Mylar® oriented polyester film, or metallic high temperaturefilm. The tape 22 has a series of holes 24 that are 2 mm in diameter andspaced about 5 mm apart. An infrared light emitting diode 26, as shownin FIGS. 1 and 2, is positioned opposite a phototransistor 28. When ahole 24 is positioned between the diode 26 and the phototransistor 28,infrared light passes from the diode to the phototransistor causing itto turn on. The use of a transmission sensor produces a more reliabledetection of tape movement which is substantially insensitive tovariation in component properties, whether variations between componentsor variations in a component due to temperature or time.

The use of infrared light is advantageous because the light is lesssubject to scattering due to dust between the light source and the lightdetector. However, other wavelengths of light could be used. As shown inFIG. 2, a comparator circuit 30 based on operational amplifier 32 isdesigned with hysteresis so that a clean digital pulse is produced foreach hole 24 that passes between the diode 26 and the phototransistor28. The comparator circuit 30 with hysteresis eliminates multiple pulsesdue to noise during the switch transition Filtering and wave shapingcircuitry may be added to further tailor the signal. The resultingoutput 34 is a digital waveform with a frequency proportional to thetape speed and the pulse width inversely proportional to tape speed.Although shown as discrete components, the circuit 30 could be on asingle chip.

Another tape 36 is shown in FIG. 4. The tape 36 is formed of transparentmaterial such as Mylar® oriented polyester film to which has beenapplied rectangular areas 38 of opaque paint or a layer ofmetallization. Metallization provides a tape 36 that has first portionswhich are electrically conductive and second portions which are notelectrically conductive serially positioned along the tape. The Mylar®film may have dimensions similar to that of the black polyethylene tape22 shown in FIG. 3, with the rectangular areas 38 being about 5 mmsquare and spaced about 5 mm apart. The tape 36 may also be used in thecartridge 20 where the transparent spaces transmit light and therectangular areas block the transmission of light.

An alternative approach of detecting a tape 36 such as the one shown inFIG. 4, in which the rectangular areas 38 are metalized, is illustratedin FIGS. 5, and 6. A tape cartridge 42 employs a fan fold tape storagetechnique with a spring biased brake 44. The tape cartridge 42 uses acapacitor based sensor 46. The sensor 46 may be used with an oscillatorcircuit 48, such as the one shown in FIG. 6, to frequency modulate abase frequency as the tape 36 passes the sensor. The oscillator circuit48 may be a simple relaxation oscillator circuit using an operationalamplifier 50 and several discrete components. It should be noted thatmany types of oscillators may be used, as long as the oscillationfrequency can be tuned by using a small capacitive element. A 555 timercircuit would be another implementation which requires no inductor.

In the circuit of FIG. 6, the tape forms a capacitive element C2 that isin parallel with the capacitor C1. This series combination of R3 and C1and C2 sets the oscillation period which may have a mid frequency ofabout 300 kHz. R1 and R2 set the threshold switching voltage. If R1=R2this voltage is ½ VCC. When the circuit is powered up, the operationalamplifier 50 rails to either the plus VCC or minus VCC output state. Theparallel combination of C1 and C2 is then charged to plus ½ VCC or minus½ VCC through the resistor R3, at which point the operational amplifierrails in the opposite direction. As the metalized area 38 on the tapeincreases the value of C2, the base frequency of the oscillatordecreases. This increase in capacitance is followed by a decrease incapacitance as the tape 36 moves to where there is no metalized area 38opposite the two plates 54 and the frequency of the oscillatorincreases. Thus an FM signal is generated which is dependent upon tapespeed. This FM signal may be demodulated to provide an output frequencycorresponding to tape speed.

The oscillator circuit 48 is based on an operational amplifier 50wherein the mid frequency of the oscillator is about 300 kHz. Thecapacitor C1 controls the frequency of the amplifier output 52. Twometal plates 54 are connected in parallel with the plates of thecapacitor C1 so that when a rectangular metalized area 38 is positionedopposite the two metal plates 54 a second capacitor C2 is formed thatincreases the capacitance of capacitor C1.

As shown in FIG. 8, yet another approach to detecting the speed of thetape 36 as it is withdrawn from the cartridge 42 is based on amplitudemodulation. An amplitude modulation circuit 56, shown in FIG. 7, has anoscillator circuit 58 that has an oscillation frequency of, for example,300 kHz to 1 MHz. The signal generated by the oscillator circuit 58 iscoupled through a capacitor C3 formed out of two metal plates 54 and ametalized area 38 on the tape 36. Thus the two metal plates 54 and themetalized area 38 of the tape form a capacitive couple between theoscillator 58 and the output 64. When the metalized area 38 completelyoverlaps the two metal plates 54, the signal is most efficientlytransmitted between the oscillator circuit 58 and the positive input 60of the operational amplifier 62 of the circuit 56. When a metalized area38 only partially overlaps the metal plates 54 or is completely absent,the transmitted signal decreases or reaches a minimum. Thus theamplitude of the signal received from the oscillator circuit 58 varieswith the speed at which the tape is moving past the capacitor C3. Theoutput 64 of the operational amplifier 62 is rectified by diode D1supplying a pulsed DC output which has frequency which is directlyproportional to the speed at which the tape 36 is being withdrawn fromthe cartridge 42.

Still another approach to detecting the speed of the tape 36 as it iswithdrawn from a cartridge 65 is based on the metalized regions 38 beingformed of a magnetically impermeable material such as iron, nickel,cobalt, or alloys based on them which have an effective amount of one ormore of the ferromagnetic metals. Mu-metal, a nickel-iron alloy (77percent Ni, 15 percent Fe, plus Cu and Mo), is particularly effective atshielding magnetic fields and also may be used. The metalized regions 38act as magnetic shunts and prevent the magnetic lines of force from apermanent magnet 66, as shown in FIG. 9, from reaching and affecting aHall effect sensor 68 which forms part of an integrated circuit whichhas a unipolar Hall sensor with the open collector output. Theintegrated Hall device 70 may perform other functions such astemperature compensation, a comparator with hysteresis, and a voltageregulator. The Hall device 70 generates a digital output when themagnetic field to which the Hall effect sensor 68 is exposed exceeds thepredetermined switch point.

Another approach to detecting the passage of the tape 36 with metalizedregions 38 is illustrated in FIG. 10. The cartridge 72 has two spacedapart electrical contacts 74 that successively engage the tape 36against a supporting member 73. When a metalized region 38 bridges theelectrical contacts 74 a circuit, not shown, provides a voltage orcurrent output which is not present when a metalized region 38 is notconnecting the contacts 74. A comparator circuit (not shown) withhysteresis removes any contact bounce and provides a clean and digitaloutput which has a frequency which is proportional to the speed at whichthe tape 36 is withdrawn from the cartridge 72.

It should be understood that the tape 22 or 36 can be used with variousmethods of storing the tape within the cartridge, for example: wrappedaround the central post, or wrapped around a rotatable spool, or simplyformed in a coil or fan fold arrangement. It should be understood thattape 22 or 36 could be a metal tape with holes formed therein. It shouldbe understood that the metallization could be by any technique whichforms a conductive film on a base film and could include plating, flamespraying, vacuum depositing, adhesive bonding, or painting theconductive regions on to a tape substrate. The tape substrate is notintended to be limited to a film but could include a woven material orfabric. Moreover, the tape material may be high temperature film, awoven cloth or any other material capable of sustaining inflatortemperatures and having the necessary tensile strength

It is understood that the invention is not limited to the particularconstruction and arrangement of parts herein illustrated and described,but embraces all such modified forms thereof as come within the scope ofthe following claims.

We claim:
 1. An airbag deployment monitor comprising: a cartridgecontaining a length of tape, the tape having a first end attached to aninside surface of an airbag cushion, wherein the tape has first portionswhich allow a passage of light, and second portions which aresubstantially opaque, and wherein the first portions alternate with thesecond portions; a light source positioned on the cartridge toilluminate one side of the tape; and a light sensor positioned on thecartridge opposite the light source to receive light when a tape firstportion is between the light source and the light sensor, the lightsensor positioned to be shielded from the light source when the tapesecond portion is between the light source and the light sensor.
 2. Theairbag deployment monitor of claim 1 wherein the light source is a lightemitting diode, and the light sensor is a phototransistor.
 3. The airbagdeployment monitor of claim 2 further comprising a comparator circuitwith hysteresis connected to the phototransistor to provide a digitaloutput each time a first tape portion alternates with a second tapeportion.
 4. The airbag deployment monitor of claim 2 wherein the lightemitting diode is of the type which emits infrared light.
 5. The airbagdeployment monitor of claim 1 wherein the tape is comprised of a blackplastic film, and the first portions of the tape define holes throughwhich light from the light source can pass.